1. Field of the Invention
The present invention generally relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus using a transfer belt for transferring an image formed on the surface of a photosensitive body (such as a photosensitive drum) to a printing medium, and a belt transfer device for use in the image forming apparatus.
2. Description of the Related Art
A typical example of an image forming apparatus that uses a transfer belt to form an image on a printing medium is a wet electrophotographic color image forming apparatus. FIG. 1 illustrates one embodiment of a conventional belt transfer device for use in a wet electrophotographic image forming apparatus.
Referring to FIG. 1, the belt transfer device 1 includes a plurality of photosensitive bodies 10 each bearing an image, and a transfer belt 20 onto which the image formed on each of the photosensitive bodies 10 is transferred.
An electrostatic latent image of printing data is formed on the surface of a photosensitive body 10 by an exposing unit (not shown), and this electrostatic latent image is developed into an image of a specific color by liquid developers supplied from a corresponding developing unit 30. For a full color image, four photosensitive bodies 10 and four developing units 30 are used to form images of four different colors. The images are transferred to the transfer belt 20 so that they are aligned.
The images formed on those four photosensitive bodies 10 are transferred and superimposed onto the transfer belt 20, and form a full color image. To make this possible, the transfer belt 20 is installed in such a manner that it follows an endless loop that makes contact with the surfaces of the respective photosensitive bodies 10. In so doing, the images formed on the surfaces of the four aligned photosensitive bodies 10 can be transferred and superimposed onto the endless loop transfer belt 20 to form a full color image. The movement of the transfer belt 20 is made possible through two rollers 21 and 23 installed on both ends. Four transfer rollers 40 are installed on the interior side of the transfer belt 20. More particularly, the four transfer rollers 40 are installed at every contact point (also called a ‘transfer nip’) with each photosensitive body 10, thereby facilitating the transfer of the images from the photosensitive bodies 10 to the transfer belt 20.
Unfortunately, however, the image forming apparatus using the belt transfer device 1 for transferring an image has a defect. When an image formed on the surface of the photosensitive body 10 is transferred onto the transfer belt 20 (that is, a T1 transfer process), the image is often dragged at a transfer nip (that is, the TI nip) where the photosensitive body 10 and the transfer belt 20 come into contact with each other. The underlying cause of the dragging of an image is the influence of a carrier liquid, which is a liquid developer at the TI nip, upon a liquid developer at a lower portion of the image. In other words, the liquid developer below the image is dragged into a non-image area, creating a defective image. FIG. 2 illustrates a typical example of the dragging (B) of an image during the T1 transfer process. As shown in FIG. 2, when the dragging occurs a printed image is blurred, or not sharp. Therefore, to obtain good quality printed materials, it is very important to prevent the dragging of an image during the T1 transfer process.
Accordingly, there is a need for an improved belt transfer device and an image forming apparatus having the same that minimizes dragging of an image during a transfer process.